What if the halfling rolled a 1 on the Charisma check you called for? You already know that the farmer is going to say yes...so how would you interpret that result of 1 given the parameters you set up?
If the PC had gotten a result of 5 or less, the farmer would have been suspicious of this stranger and kept an eye on him, and possibly even followed him around town. This would have worked out very poorly later when the PC brought a captured enemy back to the barn to interrogate him, and instead slit his throat. If the farmer had been watching, there probably would have been trouble with local law enforcement. However, because the PC did well on the check, then the farmer did not pay much attention to the goings-on in the barn, and totally shrugged off a darkness spell being used to conceal the body.
Of course, at the time of the check, I had no idea that any such hijinks could possibly ensue. It's possible the farmer would have been suspicious or friendly, and then we'd never interact with him again, and that would be that. But part of my DMing philosophy is to constantly stand up dominoes so that we can see which ones topple.